1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device and method for manufacturing the same that does not result in poor reliability due to degradation of a gate insulating material and that has stable operating characteristics.
2. Discussion of the Related Art
In a semiconductor device manufacturing process, an insulating film may serve as a surface passivation layer for shielding external impurities, as an ion implantation mask, and as an insulating film for a silicon substrate. Such insulating films greatly influence the yield of semiconductor devices. For example, with an electrical conduction channel (between source and drain) formed on a semiconductor substrate, a gate insulating film, which is used to maintain an electrical charge, has a minimum thickness for high-speed operation of a semiconductor device to maximize integration. The gate insulating film is formed on a surface of the semiconductor substrate as a thermal oxide film, i.e., by thermally oxidizing the semiconductor substrate, which is used as the gate insulating film because of its reactivity with the semiconductor substrate, carrier mobility, and interface roughness.
FIGS. 1A-1C, showing a typical semiconductor device, illustrate a conventional process for forming a gate insulating film.
As shown in FIG. 1A, an isolating film 11 is formed on a semiconductor substrate 10 having active regions and isolating regions. In this case, the isolating film 11 is formed at the isolating regions. Thereafter, a gate oxide film 12 is grown on the active regions of the semiconductor substrate 10 by performing thermal oxidation of the semiconductor substrate 10 at a high temperature. An annealing process using NO or N2O gas at a temperature higher than the growth temperature of the gate oxide film 12 is then performed on the gate oxide film 12.
As shown in FIG. 1B, a polysilicon layer is deposited on the gate oxide film 12, and is selectively etched by photolithography processes, thereby forming gate electrode 13. Thereafter, low-concentration impurity ions are implanted into the surface of the semiconductor substrate 10 at both sides of the gate electrode 13, thereby forming lightly doped drain regions 14.
As shown in FIG. 1C, an insulating film is deposited on the overall surface of the semiconductor substrate 10. Insulating film side walls 15 are formed on side surfaces of the gate electrodes 13 by an etch back process performed on the insulating film, and source/drain impurity regions 16 are formed by implanting high-concentration impurity ions into the overall surface of the semiconductor substrate 10 using the gate electrodes 13 and the insulating film side walls 15 as a mask.
In a typical method for manufacturing the semiconductor device the gate oxide film 12 is formed by thermally oxidizing the semiconductor substrate 10. The insulating characteristics of oxide film 12 are heavily dependent on the characteristics of the film. Accordingly, undesired dielectric breakdown may occur in some circumstances.
The above conventional method has several problems. For example, as the gate insulating film becomes gradually thinner, so as to form a device having a small line width, low power, and high performance, leakage current increases due to direct tunneling, thus obstructing the implementation of stable characteristics of the device. This is especially true when the SiO2 film is less than approximately 2.3 nm. In addition, as the SiO2 film is less than approximately 2.5 nm, it is difficult to perform an etching process of the gate electrodes, and PMOS gates, into which boron is implanted, have an unstable threshold voltage due to the implantation of boron.
Furthermore, as the boron present in a polysilicon layer is implanted into an oxide film during a subsequent thermal process, it deteriorates breakdown voltage of the gate insulating film.